The current state of development of electronic fuel injection systems can be attributed to both regulatory and economic influences. The regulatory influences include the imposition of strict fuel economy and pollution control laws and regulations governing the performance of motor vehicles. The economic influences include the desire to develop a fuel injection system that will accomplish the specific objectives of providing optimal engine performance in terms of fuel efficiency and pollution control, and yet be commercially feasible for production manufacture.
The focus of the present disclosure is on direct-injection, compression-ignition engines of the type used in passenger, truck, agricultural and industrial applications. In a direct-injection, compression ignition engine the critical parameters in fuel injection control are injection timing and fuel metering. The optimal values for these parameters can be affected by a number of different engine operating conditions. It is important that each significant engine operating condition that affects either of the critical parameters of injection timing or fuel quantity be measured and accounted for in controlling fuel injection.
The known existing systems for controlling fuel injection in a compression ignition engine have limitations that fall into either of two basic categories. First, such systems may be limited by the number of significant engine operating conditions to which they respond in controlling injection timing and fuel quantity. Secondly, such systems have otherwise been limited in their computational capability; i.e. by requiring that computations related to parameter values be performed in synchronism with the engine operating cycle, and thereby effectively tying the speed at which the computations can be performed to the instantaneous speed of the engine. A more specific description of each of these two categories of limitation is as follows.
The first category of limitation relates to the number and character of engine operating conditions that are used to control injection timing and fuel quantity. In mechanical fuel injection systems for compression engines, parameter values for injection advance and fuel quantity are conventionally derived on the basis of the actual engine speed and the commanded engine speed; i.e., the speed error. However, there are additional significant engine operating conditions that can materially affect these parameter values. Included among these other significant engine operating conditions are manifold air pressure, air temperature, and other variables associated with the hardware, notably the actual injectors.
The manifold air pressure and temperature determine the density of the air available for combustion in the cylinder. Air density affects directly the air-fuel (A/F) combustion ratio, and is therefore a significant factor in determining an upper limit on fuel quantity. If there is insufficient air in the cylinder to support combustion of the injected fuel, then unburned fuel will pass into the exhaust. This will adversely affect fuel economy and emissions control.
The cumulative or integral speed error is derived from the condition of the actual speed error over preceding error cycles. The cumulative speed error represents information that can be used to stabilize the steady state operation of the engine. It has proven to be particularly efficacious in stabilizing engine operation in a low speed range of 800-1000 RPM.
The injection timing can be influenced by both the quantity of fuel to be injected into the cylinder and the temperature of the air with which the injected fuel is mixed. The quantity of fuel is significant in that it will affect the timing and duration of combustion in the cylinder taken with reference to the top dead center (TDC) position. There is a predetermined angle following TDC for any given engine configuration at which it is optimal to have peak cylinder pressure to occur. It is therefore advantageous to control the timing of fuel injection as a function of the quantity of injected fuel to correlate peak cylinder pressure with the predetermined angle that corresponds to optimal power output.
For similar reasons, it is advantageous to control injection timing as a function of the temperature of the air in the cylinder. The air temperature will likewise influence the time at which combustion is initiated with reference to the TDC position. By controlling injection timing as a function of air temperature, peak cylinder pressure can be made to correspond with the angle after TDC which corresponds to maximum power output.
Another consideration in selecting the critical parameter of fuel quantity is the particular characteristics of the injector which is to inject the fuel into the cylinder. Each individual injector is a physical device that varies in performance within an acceptable range of tolerance. However, even small variations within the tolerated values can have a nonnegligible effect on fuel quantity. It is therefore desirable to have a capability to accommodate such variations in injector characteristics to make compensation in fuel quantity on the basis of these variations.
The second broad category of limitation that concerns existing fuel injection systems is computational capability. This concern is with limitation in adapting to changes in engine speed. If computations are performed in relation to engine speed or displacement, the time allowed for computations will vary depending on engine speed. Yet, it is important that sensing or measurement of engine operating condition values used in the computation of fuel injection parameter values be carried out on a regular schedule throughout the engine operating cycle. Therefore, the problem is identified as one of not unduly limiting the time for computing or otherwise processing parameter values, but on the other hand maintaining regularity or consistency in the sensing or measurement of engine operating condition values.
The present invention responds to the needs of providing expanded control capabilities in a flexible manner by the provision of an electronic fuel injection system for a compression ignition engine that meets the general objective of enhanced engine performance in terms of fuel efficiency and emissions control.